1. Field of the Invention
The present invention relates to an infrared spectrophotometer for measuring infrared absorption spectra of a sample to analyze substances in the sample qualitatively and quantitatively, and an attachment for use with the infrared spectrophotometer.
2. Description of the Background Art
An infrared spectrophotometer is designed to measure a spectrum of light transmitted through a target sample while irradiating the sample with infrared light, and determine a wavelength of light absorbed in or transmitted through the sample to analyze components of the sample.
A Fourier-transform infrared (hereinafter also referred to as “FTIR”) spectrophotometer is equipped with an interferometer in order to produce infrared coherent light. In the interferometer, an optical element, such as a beam splitter, is made of a material having deliquescent properties (properties of melting by absorbing moisture in air), such as potassium bromide (KBr). If deliquescence of the optical element occurs, it will preclude satisfactory measurements. Therefore, in order to prevent the deliquescence of the optical element, measures have been taken, for example, by placing the optical element in an element-receiving space, such as a low-humidity space gastightly isolated from ambient air while minimizing moisture vapor therein, or a vacuum space kept in a vacuumed state (see, for example, JP 2004-108970A and JU 3116465B).
During an infrared spectrophotometric measurement, if moisture vapor exists in any optical path, it will exert a negative effect on a measurement result, for example, an absorption peak of the moisture vapor will appear on measurement data, because moisture vapor has absorption peaks in a wavelength band of target substances. Thus, there is a need for additionally eliminating moisture vapor from an optical path in a device other than the interferometer, such as a sample chamber, by replacing internal air of the device with external dry air or nitrogen (performing an air-purging operation), on a case-by-case basis.
The infrared spectrophotometer is capable of measuring infrared absorption spectra of various types of gas, liquid and solid samples. The infrared spectrophotometric measurement is performed by an appropriate method selected depending on a type of sample, an intended purpose of measurement, etc. For example, the infrared spectrophotometric measurement method includes a transmission method, a reflection method, an ATR method, and various other methods.
For example, the reflection method can be used in a measurement for a substance absorbed in or applied to a material non-transmissive to infrared light, such as a metal plate. A reflection measurement for determining a reflectivity of a substance includes a method (regular reflection method or specular reflection method) in which infrared light is emitted to an entrance surface at an angle approximately perpendicular thereto, and a method (called “high-sensitivity reflection method” or “RAS (Reflection Absorption Spectrometry) method”) in which infrared light is emitted to an incidence surface at an angle approximately parallel thereto to measure a thin sample layer (thin film) on a substrate.
The ATR method can be used in obtaining absorption spectra of a smooth planar surface of a solid sample, a powder sample, or a liquid sample. ATR stands for Attenuated Total Reflection. In the ATR method, light totally reflected by a surface of a sample can be measured to obtain an absorption spectrum of the sample surface.
In the infrared spectrophotometric measurements, a plurality of types of attachments each having at least one optical element of an optical system suitable for a specific one of the various measurement method, such as an attachment for the regular reflection method (regular reflection attachment) and an attachment for the ATR method (ATR attachment), are prepared, and a measurer or operator performs the measurements while replacing between the attachments depending on an intended one of the measurement methods to form an optical system corresponding to the intended measurement method, in many cases.
An internal space of the interferometer is dehumidified so as to keep humidity at a low level in consideration of influences primarily on an optical element. However, the interferometer is designed with little regard for influences of moisture vapor on measurement data. Particularly, for example, during an operation of opening and closing a sample chamber, or an operation of replacing an attachment in the sample chamber with another one, any sections other than the interferometer, such as the sample chamber, and a pre-chamber and a detector chamber each in communication with the sample chamber, are opened to ambient air, so that moisture vapor in resulting incoming ambient air will intrude into an optical path.
Moisture (moisture vapor) not only damages the optical element but also adversely affects on infrared spectrum data. Moisture vapor has a wide absorption band, primarily, around 4000 to 3400 cm−1, 2000 to 1300 cm−1 and 400 cm−1 in a mid-infrared region. Thus, an influence of moisture vapor appears on measurement data, which is likely to cause noises in the absorption wavelength band of moisture vapor, and an undesirable situation where peaks other than those of a target sample are observed in the measurement data.
In particular, under conditions that a large amount of moisture vapor is contained in ambient air, energy in the absorption wavelength band of moisture vapor becomes smaller, and thereby the measurement data is more likely to receive a negative influence and have noises.
Further, during an operation of changing a measurement sample, it is also necessary to open and close the sample chamber and, in some cases, replace the attachment with another one depending on the measurement sample. Consequently, internal air of the sample chamber and air surrounding the attachment disposed in the sample chamber are replaced with ambient air to cause intrusion of moisture vapor into an optical path.